BUSINESS SEGMENT AND FOREIGN INFORMATION - Yen/Dollar Exchange Rates Used (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2019 USD ($) ¥ / $	Dec. 31, 2018 USD ($) ¥ / $	Dec. 31, 2017 USD ($) ¥ / $
Segment Reporting, Other Significant Reconciling Item [Line Items]
Weighted-average yen/dollar exchange rate | ¥ / $	[1]	109.07	110.39	112.16
Yen percent strengthening (weakening)		1.20%	1.60%	(3.10%)
Exchange effect on pretax operating earnings (in millions)		$ 20	$ 38	$ (63)
Yen/dollar exchange rate at December 31 | ¥ / $	[1]	109.56	111.00
Yen percent strengthening (weakening)		1.30%	1.80%
Exchange effect on total assets
Segment Reporting, Other Significant Reconciling Item [Line Items]
Exchange effect		$ 1,225	$ 1,362
Exchange effect on total liabilities
Segment Reporting, Other Significant Reconciling Item [Line Items]
Exchange effect		$ 1,533	$ 1,270

[1] Rates are based on the published MUFG Bank, Ltd. telegraphic transfer middle rate (TTM)